Method of reconstructing a high-resolution 3D image

ABSTRACT

The invention relates to a method of reconstructing a high-resolution 3D image (G) of an examination zone of a patient from a 3D image data set (D) of the examination zone, the examination zone being subject to a periodic motion which is measured, in parallel with the acquisition of the 3D image data set (D), as a motion signal (E) which represents the periodic motion. In order to enable the formation of high-resolution 3D images having an enhanced image quality, the invention proposes a method of this kind which includes the following steps:
     a) reconstructing a number of low-resolution 3D images (I) from the 3D image data set (D), the low-resolution 3D images (I) being reconstructed from 3D image data of the 3D image data set (D) which has been acquired during different phases of motion of the periodic motion,   b) determining motion information (B) of at least one sub-zone (A) of the examination zone during the different phases of motion by means of the low-resolution 3D images (I),   c) selecting a temporal reconstruction window (T) in which the motion of the at least one sub-zone (A) is below a predetermined level,   d) reconstructing a high-resolution sub-image (K) for the at least one sub-zone (A) from 3D image data (D) lying in the temporal reconstruction window (T) selected for the sub-zone (A), and   e) forming the desired 3D image (G) from the at least one sub-image (K), the zones of the 3D image (G) which are not reconstructed as sub-images then being reconstructed from the 3D image data so as to be combined with the at least one sub-image (K).

BACKGROUND

The invention relates to a method and a device for the reconstruction ofa high-resolution 3D image of an examination zone of a patient from a 3Dimage data set of the examination zone, the examination zone beingsubject to a periodic motion which is measured, in parallel with theacquisition of the 3D image data set, as a motion signal whichrepresents the periodic motion.

A method and a device of this kind are known. The reconstruction of 3Dimages of moving objects, for example, of the heart of a patient, isnowadays carried out by selecting a suitable temporal reconstructionwindow in which acceptable results can be achieved for the overallanatomy to be imaged. This means that from the 3D image data setacquired that 3D image data which has been acquired in a given timewindow in which, for example, the anatomy to be imaged has been subjectto the least motion, is selected for the reconstruction. In the case ofa periodic motion of the anatomy, for example, the cardiac motion or therespiratory motion, therefore, only 3D image data from a given phase ofmotion are used for the reconstruction, whereas all other reconstructiondata acquired is not evaluated. For the imaging of the heart or thecoronary vessels, for example, it is preferred to use exclusively 3Dimage data acquired during the diastole, whereas the formation of 3Dimages of the abdomen utilizes exclusively 3D data which has beenacquired in the state of exhalation.

The motion of given parts of organs or notably of different parts of theheart, however, does not take place simultaneously but follows theexcitation pattern of the organ; this means that individual parts of theorgan to be imaged can move to a different extent at different instants.The use of a fixed (temporal) reconstruction window for the entireobject to be imaged, therefore, often gives rise either to a degradedimage quality or to sub-optimum use of the data acquired during theassumed phases of rest of the organ.

SUMMARY

Therefore, it is an object of the invention to enhance the method andthe device of the kind set forth in such a manner that high-resolution3D images of a moving examination zone of a patient can be formed withan enhanced image quality.

This object is achieved in accordance with the invention by means of amethod as disclosed by an embodiment of the invention and by means of adevice as disclosed in another aspect of the invention. The method ischaracterized in that it includes the following steps:

a) reconstructing a number of low-resolution 3D images from the 3D imagedata set, the low-resolution 3D images being reconstructed from 3D imagedata of the 3D image data set which has been acquired during differentphases of motion of the periodic motion,b) determining motion information of at least one sub-zone of theexamination zone during the different phases of motion by means of thelow-resolution 3D images,c) selecting a temporal reconstruction window in which the motion of theat least one sub-zone is below a predetermined level,d) reconstructing a high-resolution sub-image for the at least onesub-zone from the 3D image data lying in the temporal reconstructionwindow selected for the sub-zone, ande) forming the desired 3D image from the at least one sub-image, thezones of the 3D image which are not reconstructed as sub-images thenbeing reconstructed from the 3D image data so as to be combined with theat least one sub-image.

Another embodiment of the invention discloses the corresponding devicewhich is provided with a reconstruction unit as well as an arithmeticunit in order to carry out the method.

The invention is based on the recognition of the fact that for thereconstruction of a 3D image it is less than optimum to use only 3Dimage data from the same temporal zone, that is, from a fixed phase ofmotion, for the entire examination zone, for example, an organ to beexamined such as the heart. Because individual parts of the examinationzone could move to a different extent at different instants or indifferent phases of motion, in accordance with the invention it isproposed to select specific reconstruction windows for the individualparts of the examination zone which move differently, to reconstructsub-images in such reconstruction windows, and to combine saidsub-images subsequently so as to form an overall image, that is, thedesired 3D image. Each individual sub-image can thus be derived fromdifferent 3D image data of the same 3D image set.

The complete examination zone to be reproduced in a 3D image can thus besubdivided into individual sub-zones for which a respective optimum(temporal) reconstruction window is determined each time in accordancewith the invention. However, individual sub-zones of the examinationzone which are of particular interest can also be selected in order toperform such a formation of sub-images therein on the basis of anadapted reconstruction window, whereas all other zones of theexamination zone are reconstructed in the conventional manner on thebasis of all 3D image data of the 3D image data set or on the basis of aspecial sub-quantity thereof which was acquired in a low-motion phase.

Further versions and preferred embodiments of the method and the devicein accordance with the invention are disclosed in other embodiments ofthe invention. The invention is particularly suitable for use in thereconstruction of 3D images of the heart as well as of the coronaryvessels; in that case the motion signal representing the periodiccardiac motion preferably corresponds to an electrocardiogram. Theinvention, however, in principle can also be used for the imaging ofother objects or other zones, for example, for the imaging of theabdomen or of individual organs such as the liver or the stomach,because such zones or organs are also subject to a periodic motion, thatis, the respiratory motion. A respiratory motion signal can then bemeasured as the motion signal; various methods are suitable for thispurpose, for example, a signal which represents the motion of theabdominal wall or of the diaphragm. Carrying out the invention inprinciple offers an enhanced image quality, notably when individualsub-zones or sub-objects of the examination zone to be imaged exhibit anon-simultaneous pattern of motion.

According to a preferred version of the invention, a motion model of theexamination zone is formed in order to determine the motion of the atleast one sub-zone; to this end, notably significant points in thelow-resolution 3D images are tracked or objects reproduced in thelow-resolution 3D images are segmented. Significant points may be, forexample, anatomical features such as, for example, bifurcations in thecoronary vessels. Tracking such significant points in time in theindividual low-resolution sub-images thus enables the acquisition ofmotion information on how said points have moved in time. Alternatively,this motion information can be automatically extracted by way ofsegmentation, for example, of the vascular tree in the low-resolution 3Dimages associated with various phases of motion.

According to a further preferred version a separate temporalreconstruction window is selected for each volume element of theexamination zone or for regions of the examination zone which are ofparticular interest, each volume element (voxel) or the regions ofspecial interest being separately reconstructed on the basis of the 3Dimage data acquired in the selected temporal reconstruction window. Inas far as adequate calculation time is available, or if a 3D image withthe highest possible resolution and the best possible motion correctionis to be formed, the method in accordance with the invention can thus beapplied to every individual voxel of the examination zone; this meansthat for each individual voxel there is derived motion information onthe basis of which a special temporal reconstruction window isdetermined for the voxel, that is, a window in which the reconstructionof this voxel image value takes place. However, it is also possible tocombine each time several voxels so as to form individual groups ofvoxels for which the method in accordance with the invention is carriedout so as to save calculation time.

The invention can be used in principle for the formation of a 3D imagefrom arbitrary 3D image data; this means that in principle it is notimportant which modality is used to acquire the 3D image data. Themethod in accordance with the invention, however, is preferably used inconjunction with 3D image data acquired by means of computed tomographyor by means of a 3D rotation X-ray method, for example, by means of aC-arm X-ray system.

The invention also relates to a computer program as disclosed in anotherembodiment of the invention for the execution of a method and/or forcontrolling a device as described herein. This computer program includesnotably programming means which are suitable for carrying out the methodin accordance with the invention or for controlling the device inaccordance with the invention when the computer program is executed by acomputer or a suitable arithmetic unit.

DRAWINGS

The invention will be described in detail hereinafter with reference tothe drawings. Therein:

FIG. 1 is a diagrammatic representation illustrating the method inaccordance with the invention, and

FIG. 2 shows a block diagram illustrating a device in accordance withthe invention.

DESCRIPTION

The method in accordance with the invention will be described in detailhereinafter on the basis of an example where a high-resolution 3D imageof the vascular tree of the coronary vessels is to be reconstructed.When the contraction pattern of the heart is studied, it appears thatthis pattern commences with the contraction of the atria, followed bythe contraction of the ventricles, starting with the apex. Because ofthe complex motion pattern of the anatomy observed, the various volumeelements (voxels) along the coronary artery exhibit significantlydifferent patterns of motion during the cardiac rhythm or a cardiacmotion phase. Whereas in the known reconstruction method the temporalreconstruction window is defined in a simple manner by a constanttemporal delay in respect of the R deflection in the electrocardiogram(ECG) and a given temporal length of the reconstruction window. The samereconstruction window is then used for the reconstruction of all voxelsin the volume to be reconstructed.

In accordance with the invention, however, voxel-specific reconstructionwindows are to be selected; these windows are used first for thereconstruction of individual sub-images which are subsequently combinedso as to form a desired overall image. FIG. 1 shows an electrocardiogramE which shows the characteristic variation as a function of time t of amotion signal representing the cardiac motion (a voltage is measured). Asignificant aspect of the ECG is the so-called R deflection which willbe used hereinafter so as to define the reconstruction windows.

Using an imaging system, for example, a computed tomography apparatus ora C-arm X-ray device operating on the principle of 3D rotationangiography, first a 3D image data set D is acquired. This means thatindividual data set elements, that is, so-called projections D₀₁, D₀₂, .. . , of the examination zone are acquired from different perspectivesor imaging positions, that is, continuously in time t and at equidistantinstants. These individual image data set elements together form a 3Dimage data set D wherefrom a 3D image of the examination zone can beformed. Because the acquisition of the 3D image data D takes placetemporally in parallel with the acquisition of the ECG E, the 3D imagedata D acquired can be assigned to individual phases of motion of theexamination zone. The data set elements D₀₁, D₁₁, D₂₁, D₃₁, . . . havethus all been acquired in a first phase of motion in which the ECG Eexhibits the R deflection; the data set elements D₀₂, D₁₂, D₂₂, D₃₂, . .. have been acquired in a second phase of motion with a first, fixedtemporal delay with respect to the R deflection, etc. The R deflectionsoccur at a distance in time (period) amounting to T.

In accordance with the invention respective low-resolution 3D images Iare formed from the data set elements acquired each time during the samephase of motion. This means that, for example, a first low-resolutionimage I₁ is reconstructed from the data set elements D₀₁, D₁₁, D₂₁, D₃₁,. . . of the first phase of motion, that a second low-resolution imageI₂ is reconstructed from the data set elements D₀₂, D₁₂, D₂₂, D₃₂, . . .of the second phase of motion, etc. Each of these low-resolution imagesI₁, I₂, I₃, . . . is thus formed from 3D image data exhibiting a fixedtemporal delay relative to the R deflection.

From such low-resolution 3D images I₁, I₂, I₃, . . . there is derivedmotion information B which provides information on the motion of theanatomy present in the examination zone during the cycle of motion. Inthe example described herein, therefore, motion information B is to beacquired as to how individual zones of the heart or individual coronaryvessels move during the cardiac cycle. To this end, for example,significant points such as bifurcations of the coronary vessels can betracked in the individual low-resolution images I or the vascular treecan be automatically segmented in these images. It is thus possible toderive information defining the phases of motion in which sub-zones ofthe examination zone, or in an extreme case individual voxels of theexamination zone, have moved and to what extent and also the phases ofmotion or periods of time in which no or only insignificant motion hasoccurred during a motion cycle.

This motion information is subsequently used to determine a respectiveoptimum temporal reconstruction window T₁, T₂, T₃, . . . for individualsub-zones A₁, A₂, A₃, . . . , which may also correspond to individualvoxels in an extreme case. For a first sub-zone A₁ a first temporalreconstruction window T₁ is thus determined; this means at the same timethat only image data set elements acquired from this segment of time ofthe motion cycle are used for the reconstruction of a high-resolutionsub-image K₁ of this sub-zone A₁. This reconstruction window T₁ can alsobe recognized in FIG. 2; it can be deduced therefrom that, for example,the reconstruction of a high-resolution sub-image K₁ of the sub-zone A₁utilizes only the image data set elements D₀₂, D₀₃, D₀₄, D₀₅, D₁₂, D₁₃,D₁₄, D₁₅, D₂₂, D₂₃, . . . because this sub-zone obviously has not beensubject to motion, or only hardly so, during the time window T₁ of themotion cycle. For a second sub-zone A₂, however, a different temporalreconstruction window T₂ is derived from the motion information B. Forthe formation of a high-resolution image K₂ of the sub-zone A₂,therefore, image data set elements D₀₄, D₀₅, D₀₆, D₀₇, D₁₄, D₁₅, . . . .are used, because obviously the sub-zone A₂ has moved the least duringthe segment of time T₂.

For each sub-zone A for which the method in accordance with theinvention is to be used, therefore, an individual temporalreconstruction window T is selected, the temporal delay relative to theR deflection and the duration in time of the reconstruction window maybe different for each sub-zone A. The reconstruction windows, however,may overlap or also be identical.

As has already been described, in an extreme case a separate temporalreconstruction window T can be determined for each individual voxel ofthe examination zone, so that each individual voxel is optimallyreconstructed individually from specially selected data set elements. Inanother extreme case a separate reconstruction window may be determinedfor a single sub-zone only of the examination zone, whereas all otherzones of the examination zone are reconstructed in a customary mannerfrom all 3D image data of the 3D image data set D or from 3D image dataof a fixed temporal reconstruction window. It is to be noted again thatthe reconstruction windows are variable and cover for each sub-zone adifferent length and a different segment in time of the motion period.

In a final step the high-resolution sub-images K₁, K₂, K₃, . . . formedare combined so as to form the desired high-resolution 3D image G. Thesub-zones in this 3D image for which a separate reconstruction windowwas selected in accordance with the invention thus exhibit a clearlyenhanced image quality with a higher resolution and fewer artifacts.

FIG. 2 is a representation in the form of a block diagram of a device inaccordance with the invention for carrying out the described method.Therein, first a 3D image data set D is acquired by means of an imagedata acquisition device 1, for example, a computed tomography, apparatusor a C-arm X-ray device. In parallel therewith a motion signal E, forexample, an ECG is acquired by means of a suitable measuring device 2,for example, by means of an electrocardiograph; this motion signalprovides information as regards the motion of the anatomy to be imaged.The data acquired is first stored in a memory 3 and subsequently appliedto the reconstruction unit 4 for evaluation and further processing.

Therein, the low-resolution 3D images I, representing the examinationzone in different phases of motion, are formed first in a firstreconstruction module 41. Subsequently, said low-resolution images areapplied to an arithmetic unit 5 in which the motion information B, forexample, in the form of a motion model, is acquired in a first module51; this motion information is subsequently evaluated in a second module52. In the module 52, therefore, the motion information is used todetermine the individual temporal reconstruction windows T forindividual sub-zones A which are defined, for example by a user. Thesereconstruction windows are then applied to a second reconstructionmodule 42 which forms the respective high-resolution sub-images K forthe individual sub-zones A on the basis of this information and as wellas on the basis of the motion signal E from the 3D image data set. Thesehigh-resolution sub-images K are ultimately combined in a combinationmodule 43 so as to form the desired high-resolution 3D overall image Gof the examination zone.

1. A method of reconstructing a high-resolution 3D image (G) of anexamination zone of a patient from a 3D image data set (D) of theexamination zone, the examination zone being subject to a periodicmotion which is measured, in parallel with the acquisition of the 3Dimage data set (D), as a motion signal (E) which represents the periodicmotion, which method includes the steps of: a) reconstructing a numberof low-resolution 3D images (1) from the 3D image data set (D), thelow-resolution 3D images (I) being reconstructed from 3D image data ofthe 3D image data set (D) which has been acquired during differentphases of motion of the periodic motion, b) determining motioninformation (B) of at least one sub-zone (A) of the examination zoneduring the different phases of motion by means of the low-resolution 3Dimages (I), c) selecting a temporal reconstruction window CT) in whichthe motion of the at least one sub-zone (A) is below a predeterminedlevel, d) reconstructing a high-resolution sub-image (E) for the atleast one sub-zone (A) from 3D image data (D) lying in the temporalreconstruction window (T) selected for the sub-zone (A), and e) formingthe desired 3D image (G) from the at least one sub-image (K), the zonesof the 3D image (G) which are not reconstructed as sub-images then beingreconstructed from the 3D image data so as to be combined with the atleast one sub-image (K).
 2. A method as claimed in claim 1, wherein amotion model of the examination zone is formed in order to determine themotion information (B) of the at least one sub-zone (A), that is,notably by tracking significant points in the low-resolution 3D imagesor by segmentation of objects reproduced.
 3. A method as claimed inclaim 1, wherein for each voxel of the examination zone, or for regionsof the examination zone which are of particular interest, there isselected a separate temporal reconstruction window (T), and that eachvolume element or each region of particular interest, is separatelyreconstructed on the basis of the 3D image data acquired in the selectedtemporal reconstruction window (T).
 4. A method as claimed in claim 1,wherein the 3D image data set (D) has been acquired by means of acomputed tomography apparatus or a 3D rotation X-ray device.
 5. A methodas claimed in claim 1, wherein an electrocardiogram or a respiratorymotion signal is used as the motion signal (E) representing the motionof the examination zone.
 6. A computer program stored on a computerreadable medium for carrying out the method claimed in claim
 1. 7. Adevice for reconstructing a high-resolution 3D image (G) of anexamination zone of a patient from a 3D image data set (D) of theexamination zone, the examination zone being subject to a periodicmotion which is measured, in parallel with the acquisition of the 3Dimage data set (D), as a motion signal (E) which represents the periodicmotion, which device includes a) a reconstruction unit (4) for thereconstruction of a number of low-resolution 3D images (I) from the 3Dimage data set (D), the low-resolution 3D images (I) being reconstructedfrom 3D image data of the 3D image data set (D) which has been acquiredduring different phases of motion of the periodic motion, b) anarithmetic unit (5) for determining the motion of at least one sub-zone(A) of the examination zone during the different phases of motion bymeans of the low-resolution 3D images (1), the arithmetic unit beingarranged to select a temporal reconstruction window (T) in which themotion of the at least one sub-zone (A) is below a predetermined level,and the reconstruction unit (4) being arranged to reconstruct at leastone high-resolution sub-image (K) for the at least one sub-zone (A) from3D image data (D) lying in the temporal reconstruction window (T)selected for the sub-zone (A), and to form the desired 3D image (G) fromthe at least one sub-image (K), the zones of the 3D image (G) which arenot reconstructed as sub-images then being reconstructed from the 3Dimage data so as to be combined with the at least one sub-image (K). 8.A device for forming 3D images of an examination zone of a patient whichincludes an image acquisition device (1) for the acquisition of a 3Dimage data set of the examination zone, the examination zone beingsubject to a periodic motion, and also includes a measuring device (2)for measuring, in parallel with the acquisition of the 3D image dataset, a motion signal which represents the periodic motion, and also adevice for the reconstruction of a high-resolution 3D image as claimedin claim
 6. 9. A device as claimed in claim 8, wherein the device is acomputed tomography apparatus or a 3D rotation X-ray device, notably aC-arm X-ray device.